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Piston oil cooling process for marine diesel engine

In an internal combustion engine Piston is the cylindrical metal item which reciprocates within a cylinder. It may move as a result of fluid pressure as in an engine, or it can compress a fluid, as in a pump or compressor. The piston consists of piston crown and piston skirt screwed together. Piston crown or piston head is the upper part of a piston exposed to hot gases in an engine.

Piston ring is a rectangular cross-section ring of cast iron which is cut to enable fitting over a piston. The ring fits into a groove and creates a gas tight fit of the piston when moving up and down in the cylinder.

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Piston oil cooling process: The piston absorbs heat of combustion and this heat must be conducted away if the metal temperature is to kept in safe limits.

The coolant used for removing and conveying the heat from a piston may be either fresh water, distilled water or lubricating oil. Water has the ability to remove more heat than lubricating oil ( specific heat of water approximately 4 and lubricating oil 2 and temperature difference 14 deg C for water and 10 deg C for lube oil)

Modern engines have oil cooled pistons. The piston rod is utilised to carry the oil to and from the piston. The rod is hollow, and has a tube running up its centre. This gives an annular space which, with the central bore, allows a supply and return.

Oil cooling of piston is achieved by various means; The simplest is for a jet of oil to be directed upwards from a hole in the top of the con rod onto the underside of the crown. The oil is sprayed up matching bores onto the underside of the crown. This allows the crown to be made as thin as possible, to allow for maximum heat transfer while maintaining strength, and combined with the "cocktail" shaker effect caused by the reciprocating motion, gives efficient cooling.

Piston cooling

Fig:Path of piston cooling

A more efficient method is to use an oil catcher as shown in the picture . This directs oil into the cooling spaces on the underside of the crown where the cocktail shaker effect of the reciprocating piston ensures a positive cooling effect. It is unusual for the oil return temperature to be monitored (unlike the 2 stroke slow speed crosshead engine, where both temperature and quantity are monitored).

An alternative method of cooling uses a nozzle plate and nozzles. Note that the oil goes up the annular space formed between the oil tube and the bore in the piston rod, and returns down the centre.

Piston crown

Fig:Piston crown


Summarized below more guidance for marine diesel engine piston handling safety procedure:
  1. Construction of 4 stroke piston

    2. For medium and high speed engines the weight of the material becomes important to reduce the stresses on the rotating parts. The high thermal conductivity of aluminium alloys allied to its low weight makes this an ideal material. To keep thermal stresses to a reasonable level cooling pipes may be cast into prevent distortion.
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  2. Piston oil cooling method - basic principle

    3. The coolant used for removing and conveying the heat from a piston may be either fresh water, distilled water or lubricating oil. Water has the ability to remove more heat than lubricating oil ( specific heat of water approximately 4 and lubricating oil 2 and temperature difference 14 deg C for water and 10 deg C for lube oil) .

    More .....

  3. Function of piston ring in a marine diesel engine

    4. The efficiency of the engine depends upon the effective sealing between the piston and liners. Leakage will reduced compression pressure and power will lost. Piston rings seal the gas space by expanding outwards due to the gas pressure acting behind them. They also spread the lubricating oil up and down the cylinder liner and transfer heat to the liner walls Three to six power or compression rings are fitted to the piston, the number depending on weather the engine operates on the 2-stroke or 4-stroke cycle.

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  4. Requirement of piston ring

    5. Piston forms the lower part of the combustion chamber in a marine diesel engine. It seals the cylinder and transmits the gas pressure to the connecting rod.

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  5. Piston designing considerations

    6. Piston forms the lower part of the combustion chamber in a marine diesel engine. It seals the cylinder and transmits the gas pressure to the connecting rod.

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    Troubles of piston to be avoided
    i) Piston seizure; all too often followed by an explosion of the lubricating oil – air mixture in the crankcase. ii) Cracking of piston crown and side walls, due to cyclic variation of pressure stress superimposed on stress due to temperature gradient.

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  6. Detection of piston overheating

    7. most common reasons of Piston running hot : i) Inadequate circulation of cooling media and or supply not sufficient. ii) Excessive deposit in cooling space (scale or carbon). iii) Lubrication not sufficient.

    More .....









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